In recent years, there has been energetically studied employment of solar light which is inexhaustible and causes no toxic material. Practical uses of solar light as a clean energy source include inorganic solar cells for residential use, such as a single crystalline silicon, a polycrystalline silicon, made of an amorphous silicon, cadmium telluride, and indium selenide.
Shortcomings of inorganic solar cells, for example, are that silicon solar cells require extremely high purity is required, which complicates purification process and results in enhanced production cost.
On the other hand, there are a number of solar cells employing organic material. Organic solar cells include a Schottky type photoelectric conversion element in which a p-type organic semiconductor and a metal exhibiting a small work function are joined, and a hetero-junction type photoelectric conversion element in which a p-type organic semiconductor and an n-type inorganic semiconductor, or a p-type organic semiconductor and an electron-accepting organic compound are joined. There are employed organic semiconductors such as chlorophyll, synthetic dyes and pigments, e.g., perylene, electrically conductive polymeric materials and their composite materials. These materials are thin-layered through a vacuum deposition, casting or dipping method to constitute cell materials. Organic materials have advantages such as low-cost and large dimensions being readily achieved but also have problems in that almost all of them exhibit a conversion efficiency of not more than 1% and their durability is inferior.
In such a situation, a solar cell exhibiting superior characteristics was reported in B. O'Regan & M. Gratze, Nature, 353, 737 (1991). The proposed cell is a dye-sensitized solar cell and is also a wet solar cell having a working electrode of a porous titanium oxide thin-layer, spectrally sensitized with a ruthenium complex. Advantages of this system are that it is not necessary to purify a low-priced metal compound semiconductor such as titanium oxide to high purity so that it is low-cost and can employ lights extending over the broad visible region and a solar light having a large visible light content can be effectively converted to electricity.
On the other hand, a resourceless ruthenium complex is used and supply of the ruthenium complex is a concern for its practical use. The ruthenium complex has problems in that it is expensive and unstable in aging. Therefore, if it is replaced by an inexpensive and stable dye, these problems are largely overcome.
It was also disclosed that the use of a compound having a triphenylamine structure as a dye for a cell achieved an element exhibiting an enhanced photoelectric conversion efficiency, as described in JP-A No. 2005-123033 (hereinafter, the term JP-A refers to Japanese Patent Application Publication). However, such a dye was proved to have problems that adhesion to titanium oxide was low, enhanced sensitization effect was not achieved and durability was also inferior.